In simple terms
A friendly intro before the formal notes — no formulas yet.
Organic Reactions: The Basic Patterns
Different families of organic molecules have predictable ways of reacting, known as characteristic reactions. By understanding these core patterns—substitution, addition, and oxidation—we can predict how molecules will transform.
Think of functional groups as different types of Lego bricks. A flat 2x4 brick (an alkene) has studs on top where you can add other bricks (electrophilic addition). A brick with a special clip (a halogenoalkane) allows you to swap that clip for another type (nucleophilic substitution). You can't just connect any brick anywhere; you must follow the specific connection rules for each type.
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Free-radical substitution: Alkanes are unreactive, but UV light can break a halogen molecule into highly reactive free radicals, starting a chain reaction that substitutes a hydrogen atom.
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Electrophilic addition: The electron-rich double bond in an alkene attracts electrophiles (electron-pair acceptors), breaking the pi bond to form new single bonds as the electrophile adds across the double bond.
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Nucleophilic substitution: The polar C-Halogen bond in a halogenoalkane is attacked by a nucleophile (electron-pair donor), which replaces the halogen atom. The mechanism (Sₙ1 or Sₙ2) depends on the halogenoalkane's structure.
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Oxidation: Alcohols can be oxidised using an oxidising agent like acidified potassium dichromate(VI). The product depends on whether the alcohol is primary, secondary, or tertiary, and the reaction conditions used.
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Key formulas
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$\xrightarrow[distil]{[O]}$
Full topic notes
Formal explanation with the rigour you need for the exam.
1. Free-Radical Substitution in Alkanes
Alkanes are saturated hydrocarbons with strong, non-polar C-C and C-H bonds. This makes them relatively unreactive. However, in the presence of ultraviolet (UV) light, alkanes will react with halogens, such as chlorine or bromine, in a free-radical substitution reaction. The UV energy provides the activation energy needed to break the halogen-halogen bond.
Initiation: The reaction begins when UV light causes homolytic fission of a halogen molecule, creating two highly reactive halogen free radicals. e.g.,
Propagation: This is a two-step chain reaction. A chlorine radical attacks an alkane, forming HCl and an alkyl radical (). This alkyl radical then attacks another chlorine molecule, forming the product and regenerating a chlorine radical ().
Termination: The reaction stops when two free radicals collide and combine, removing radicals from the system. e.g., or or .
Free-radical substitution is not a 'clean' reaction. It produces a mixture of products because substitution can occur at any position on a longer alkane chain, and multiple substitutions can occur on the same molecule (e.g., forming dichloromethane, trichloromethane, etc.).
2. Electrophilic Addition in Alkenes
The defining feature of an alkene is the C=C double bond. This bond consists of a strong sigma (σ) bond and a weaker pi (π) bond. The π-bond is a region of high electron density located above and below the plane of the sigma bond, making it vulnerable to attack by electrophiles (electron-pair acceptors).
3. Nucleophilic Substitution in Halogenoalkanes
In a halogenoalkane, the carbon-halogen bond (C-X) is polar because halogens are more electronegative than carbon. This creates a permanent dipole, . The electron-deficient carbon atom is susceptible to attack by nucleophiles (electron-pair donors), which replace the halogen atom in a substitution reaction.
Sₙ2 Mechanism (Bimolecular): Favoured by primary halogenoalkanes. A single, concerted step where the nucleophile attacks the carbon at the same time as the C-X bond breaks. The rate depends on the concentration of both the halogenoalkane and the nucleophile. e.g.,
Sₙ1 Mechanism (Unimolecular): Favoured by tertiary halogenoalkanes. A two-step process. First, the C-X bond breaks heterolytically to form a stable tertiary carbocation (this is the slow, rate-determining step). Second, the nucleophile rapidly attacks the carbocation. The rate depends only on the concentration of the halogenoalkane.
Reactivity Trend: The reactivity of halogenoalkanes in nucleophilic substitution increases down the group: C-I > C-Br > C-Cl > C-F. This is because the C-X bond enthalpy decreases, making the bond easier to break.
4. Oxidation of Alcohols
Alcohols can be oxidised by a suitable oxidising agent, most commonly acidified potassium dichromate(VI), . The product of the oxidation depends on the class of the alcohol (primary, secondary, or tertiary) and the reaction conditions.
Primary Alcohol $\xrightarrow[distil]{[O]}$ Aldehyde $\xrightarrow[reflux]{[O]}$ Carboxylic Acid
Secondary Alcohol $\xrightarrow[reflux]{[O]}$ Ketone
Tertiary Alcohol No reaction
Primary Alcohols: Can be oxidised twice. Gentle heating and immediate distillation yields an aldehyde. Strong heating under reflux provides enough energy for the full oxidation to a carboxylic acid.
Secondary Alcohols: Can be oxidised once to form a ketone. Ketones are resistant to further oxidation under these conditions, so reflux can be used without issue.
Tertiary Alcohols: Do not undergo oxidation. This is because the carbon atom attached to the -OH group has no hydrogen atoms to be removed. Breaking a C-C bond would be required, which needs much harsher conditions.
Observation: A positive test for oxidation is the colour change of the dichromate solution from orange () to green ().
For primary alcohol oxidation, the conditions are crucial. If a question asks for the aldehyde, you MUST mention distillation. If it asks for the carboxylic acid, you MUST mention reflux. These keywords are often worth a mark.
Worked examples
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Predict the major product when propene () reacts with hydrogen bromide (). Outline the mechanism and explain your reasoning.
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This is an electrophilic addition reaction. The HBr molecule is polar, .
1-chlorobutane is heated under reflux with an excess of aqueous sodium hydroxide. Name the organic product and the type of reaction mechanism.
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The reactant is 1-chlorobutane (), a primary halogenoalkane. The reagent is aqueous sodium hydroxide, which provides the hydroxide ion (), a strong nucleophile.
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What is a free radical?
A species with an unpaired electron, typically formed by homolytic fission of a covalent bond. It is highly reactive.
Key takeaways
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Initiation: The reaction begins when UV light causes homolytic fission of a halogen molecule, creating two highly reactive halogen free radicals. e.g.,
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Propagation: This is a two-step chain reaction. A chlorine radical attacks an alkane, forming HCl and an alkyl radical (). This alkyl radical then attacks another chlorine molecule, forming the product and regenerating a chlorine radical ().
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Termination: The reaction stops when two free radicals collide and combine, removing radicals from the system. e.g., or or .
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